Najib Razak’s government has clamped down on opposition after questions emerged over the management of a state investment fund

KUALA LUMPUR, Malaysia—Tens of thousands of protesters gathered in Kuala Lumpur on Saturday to demand the resignation of Malaysia’s Prime Minister Najib Razak, the day after police banned the protest and arrested two of the activists who planned it.

The arrests came hours after police and regulatory officials on Friday raided the office of the activist group known formally as the Coalition for Clean and Fair Elections, but better...